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Abstract

The proportion of guanine (G) or cytosine (C) nucleotide bases in the human genome is approximately 40%. Fluctuating relatively to this averaged G+C content, there are large chromosomal regions with alternatingly high and low local G+C contents. The (G+C)-rich genomic regions are typically gene rich, replicated earlier and higher in recombination activities. Many aspects of the evolution of base composition leading to the local G+C variations in the human genome have been debated, including the origin of the alternating G+C structure, whether (G+C)-rich regions have been vanishing or emerging, what are the main mutational or nonmutational mechanisms driving the G+C content, whether G+C content is determined by neutral evolution or selection or both. A particularly promising scenario for G+C evolution is to consider the open and closed chromatin structures which naturally provide a differential mutational environment for different chromosome regions.

Key Concepts:

Human genome is characterised by alternatingly high and low G+C regions.

Such pattern of alternatingly high and low G+C regions is not unique for the human genome. It is also present in species which diverged from a common ancestor a few hundred millions years ago.

Both mutational and nonmutational mechanisms could be responsible for shaping the G+C content in human. The former creates new alleles with a changed G+C content, the latter biased transmits existing polymorphic alleles towards either higher or lower G+C.

The suggestion of natural selection playing a crucial role in maintaining high G+C regions is based on the observation that high G+C regions are also high in several biological activities (translation, transcription and recombination).

Chromatin structure may provide a differential mutational environment by being accessible (open) or inaccessible (closed) to other protein molecules.